The present invention relates to smoke detection apparatus, and more particularly, to apparatus for detecting the presence of smoke or other solids under a variety of conditions.
It is a well-known fact that early and reliable indication of the presence of smoke in a building or the like can do much to save lives inasmuch as a tremendous number of persons are killed due to smoke inhalation prior to the actual consumption of the portion of the building structure in which the victims happen to be located.
A variety of apparatus has been proposed in the prior art for detecting the presence of smoke, it being a common practice to provide apparatus including an exciter lamp which functions to illuminate a dark space in which particulate matter such as smoke is to be detected. Also included as part of such apparatus is a pair of photocells which operate to view the enclosed space. The reason that a pair of photocells is conventionally utilized is that a photocell's reponse to light reflected or scattered from the particulate matter in the smoke being detected varies with age, temperature, applied voltage and change of lamp illumination, as well as other factors. Accordingly, the pair of photocells is incorporated in a balanced circuit arrangement such that the aforesaid deleterious effects tend to cancel out.
In order to provide a representative sample of prior art schemes for smoke detection so as to furnish background for the subject matter of the present invention, reference may be made to U.S. Pat. No. 3,409,885, and also to U.S. Pat. No. 3,723,747 and U.S. Pat. No. 3,727,056.
Whatever the merits of the various schemes and systems heretofore proposed, it has been recognized by the present inventors that a nunmber of serious drawbacks are involved. For example, although certain kinds of balanced-circuit arrangements help to alleviate problems associated with a photocell response, yet such arrangements have not been entirely satisfactory in that any imbalance in the light intensities between the two photocells leads to a disparity between the sensitivity of the two cells after an extended period of time.
A particularly troublesome set of difficulties are those presented when an incandescent light source is to be utilized as the source to be monitored within the smoke detection chamber. As is well known, such a source has a relatively short useful life of the order of several years. Moreover, replacement in service is quite often required and, in fact, when used as the light source in a smoke detector, a replacement lamp is often included as part of the purchased unit. An incandescent light source also generates an excessive amount of heat and "noise"; that is to say, it generates such a high degree of illumination that much of the light is scattered throughout the enclosure and is not absorbed by the blackened interior thereof.
It is therefore a primary object of the present invention to provide a smoke detection device or apparatus that can successfully exploit the inherent advantages of a solid state, low illumination level lamp as the light source. Such a light source has an extremely long life of over 20 years and therefore should not need replacement.
A further object is to keep the power dissipation requirements on such a solid state source to a minimum by operating the source at an extremely low level of illumination and therefore a low level of current.
Yet another object of the present invention is to permit such low level of illumination, and yet to realize or produce the required sensitivity even though the illumination received by the photocells under smoke conditions varies only slightly from the normal illumination. The sensitivity requirement on a smoke detector is such that it must respond to visible smoke obscuration of light of approximately 2% per foot (0.01 optical density). This requirement or specification is satisfied by the present invention with as small a change in illumination of the photocells as 10% of the normal total illumination. By normal total illumination is meant the illumination received by both photocells under normal conditions, that is when no smoke is present in the detection chamber.
A further object is, through the use of a solid state light source, to provide a small, portable, inexpensive and reliable unit that can be employed in the home and which will not require frequent servicing.
Another object of the invention is to provide a smoke detector which insures that a significant flow of air will constantly be maintained through the detector so that the smoke content can be continuously monitored.
A further requirement, which must be fulfilled for satisfactory and reliable operation of a smoke detection device, is that if the device is to include a solid state light source having a low level light output, the device must be able to respond and to give an alarm when the smoke possesses widely different characteristics. Thus, the smoke detector must be highly sensitive to the presence of either so-called white smoke or black, that is non-reflective, smoke.
Accordingly, it is another object of the present invention to provide a smoke detection device that will respond reliably to widely disparate smoke characteristics even though the light illumination is at relatively low level.
The above and other objects of the invention are fulfilled and implemented by a primary feature of the present invention which provides apparatus for detecting the presence of smoke, such apparatus comprising a housing defining an enclosure for the detector; a detection chamber within the enclosure for permitting a continuous air flow so that smoke present therein may be monitored; a solid state low level light source mounted within said enclosure; first and second light receiving means mounted within the enclosure (the first means being preferably arranged to receive approximately 70% of the total normal illumination received by both measuring means and the second means arranged to receive approximately 30%); a sensing circuit, connected to a threshold means, said first and second light receiving means, the sensing circuit being operable to actuate the threshold means whenever there is a change in the illumination received by either of said light receiving means as small as 10% of the normal total illumination; and alarm signal means electrically connected to said threshold means and actuated thereby to produce an alarm signal.
The first light receiving means mentioned above is constituted by a first photocell, termed a direct photocell because it is in the direct path of illumination from the light source. This direct photocell is predominantly responsive to black smoke, the conductivity of this photocell decreasing significantly as the presence of black smoke increases. The second light receiving means is constituted by another photocell, termed the "reflected" photocell, which is predominantly responsive to white smoke, the conductivity of this photocell increasing significantly as the presence of white smoke increases.
The pair of photocells referred to above is connected in a sensing circuit so as to initiate an alarm when any of the following conditions occurs: (a) black smoke is present in the smoke chamber whereby the conductivity of the direct photocell decreases, or (b) white smoke is present such that the conductivity of the indirect photocell increases, or (c) a mixture of black and white smoke in variable proportions is present in the smoke chamber such that the conductivity of the direct photocell decreases and the conductivity of the indirect photocell increases.
The sensing circuit is connected, as noted previously, to the alarm threshold means which is activated only when a predetermined difference in potential at the output of the sensing circuit, due to the disparity from the normal conductivity in the two photocells, exceeds a predetermined value. The alarm threshold means includes a programmable unijunction transistor (PUT) switch which actuates an alarm device upon its being switched by the sufficiently great potential difference at its input.
Under normal circumstances, that is, when no smoke is present within the detection chamber, the differential voltage applied at that time between one of the main electrodes and the gate electrode of the unijunction transistor has a value of zero volts. Thus, the normal illumination received by both cells is suitably proportioned such that this zero differential is obtained.
The particular adjustment of the light source so as to produce the aforesaid proportioning of the illumination constitutes a further feature of the present invention. By this feature the illumination received by the direct cell is initially arranged to be approximately 2.5 times the illumination received by the reflected cell. Then, instead of adjusting a separate resistance in a circuit with the photocell, the light beam axis is changed slightly. This has been found to be an efficacious adjustment; and one that eliminates several difficulties, one of which is that the neutral density filter transmission becomes less critical. Also, the match required between photocell resistances becomes less critical, as will be made apparent as the description proceeds. This lessening of the criticality of the photocell resistances results from the fact that these are being effectively determined by the degree of illumination received due to the precisely adjusted setting of the light beam axis.
In accordance with another feature of the present invention the amount of light impinging on the direct photocell is reduced by a single neutral filter having a light transfer of 0.1% to 0.5%.
Yet another feature of the present invention resides in the provision of reflective strips, associated with a heat generating resistor, all being located near the air flow outlet from the detector device. Such an arrangement enables the detector to operate with efficient air flow whether in a position resulting from being mounted on the ceiling, or from being mounted on a wall.
A further feature of the present invention resides in a unique arrangement for providing a "trouble" signal when either or both of the light producing devices in the detector have failed. This arrangement utilizes the same device that serves to indicate the presence of smoke, but does so with a markedly different audible signal.
The above objects, features and advantages of the present invention will be understood, as well as additional aspects thereof, by reference to the following detailed description of a preferred embodiment and to the accompanying drawing, in which: